Hi, Peter!
First of all I'd like to congratulate you for this awsome iniciative.
Like someone has said on the forum, WireframeSketcher has a powerful potential because it's eclipse based. A little bit of work should lead it to the top of wireframes tools.
Some folks on the forum asked prototyping features, but I think you should aim only wireframes, like OmniGraffle does. In my opinion it's the best software for this porpouses.
If you want some tips of the top features and aspects of it , count on me to help. I'm not a Java developer, but maybe I can be helpful...
Thanks!